Contemporary navigation systems commonly employ satellite navigation data derived from radio navigation receivers to determine the location of a navigation platform. The satellite data is often augmented by data from additional sensors in order to improve navigation system performance in situations where satellite data may be available only intermittently or may be degraded by intentional or unintentional interference. For example, data from inertial sensors (accelerometers and gyros) may be used to allow full navigation solutions (position, velocity and attitude) to be maintained in the presence of satellite data dropouts over extended periods of time. Radio navigation data is generated by receivers specifically designed to receive and process radio signals transmitted from a plurality of space-based or ground-based terminals. The navigation sensors are generally, but need not be, collocated on the same navigation platform. Depending on the application, the navigation system objective may be to determine position only, position and velocity, or a larger set of parameters which may include navigation platform attitude. If required, the satellite navigation data can be used to perform error calibration of the other sensors, thereby reducing navigation system errors when the satellite data are temporarily unavailable or corrupted by interference.
Radio navigation data are processed in order to determine the line-of-sight distance of the navigation base from each transmitter in view of the receiver. This is accomplished by measuring the time of arrival of each signal, and comparing it to the known time of transmission, using a common time reference. Contemporary radio navigation receivers used for this purpose generally include an antenna for receiving the radio signals, a front-end for amplifying, down-converting and bandpass-filtering the received signals, an analog-to-digital converter, and a signal processor. It is common practice in such receivers to generate in-phase (I) and quadrature (Q) baseband signals, which are then processed to remove noise and interference.
Noise and interference are often reduced significantly by means of spread spectrum techniques in which the transmitted signal is modulated by a known pseudo-random code. The receiver correlates the received signal with a locally-generated replica of the code and performs code tracking by varying the estimated time delay to maintain the correlation at or near its peak value. In this manner, a significant improvement in signal-to-noise ratio is obtained. If the input signal-to-noise ratio is sufficiently high, then it is possible to perform carrier phase tracking. Successful carrier tracking yields navigation accuracies which are greater than those achievable using code tracking only.
The NAVSTAR Global Positioning System (GPS), developed by the United States Government, is an example of a contemporary radio navigation system. A constellation of up to 24 satellites, positioned in precisely-known orbits, transmit pseudo-random ranging signals used by specially-designed receivers to calculate line-of-sight distance from the user receiver to any satellite in view of the receiver. Based on this information, a navigation solution can be obtained.
In modern GPS-based navigation systems, interference can have adverse effects on GPS receiver code and carrier tracking, resulting in degraded navigation system performance. Interference can be intentional or unintentional. Examples of unintentional interference include: (1) out-of-band signals from nearby transmitters with inadequate radio-frequency (rf) filtering, (2) harmonic or intermodulation products of various ground and airborne transmitters, (3) active or passive intermodulation products of signals or local oscillators on the same platform as the GPS receiver or on nearby platforms, (4) pulsed interference from radar signals in nearby frequency bands, (5) accidental interference from unlicensed transmitters. The results of interference are a reduction in signal-to-noise ratio (SNR) at the receiver input. The transmitted GPS signal may be further attenuated by trees, buildings, etc., resulting in a further reduction of SNR. Sources of intentional interference include narrowband and wideband jammers specifically designed to reduce SNR at the receiver input.
Current GPS satellites transmit on two frequencies, L1=1575.42 MHZ and L2=1227.6 MHZ. The satellites transmit their signals using spread spectrum techniques and employ two different spreading functions: (1) a 1.023 MHZ coarse/acquisition (C/A) code on L1 only and (2) a 10.23 MHZ precision P(Y) code on both L1 and L2. The minimum signal power for received GPS signals is specified as follows; for L1, C/A=-160 dBW (decibels with respect to one Watt), P=-163 dBW; for L2, P=-166 dBW. A typical value of equivalent received thermal noise power is -131 dBW. Thus, recovery of the GPS signal, even with no interference, cannot be accomplished without special design techniques such as spread spectrum.
As described above, the GPS signal is broadcast using standard spread spectrum techniques in which the narrow bandwidth signal is spread to a much larger bandwidth by using a pseudo-random code. By correlating the received signal with a known replica of the pseudo-random code and then bandpass filtering the result over the narrow signal bandwidth, the effects of interference are significantly reduced. The gain in SNR due to spread spectrum processing is on the order of 53 dB for P(Y)-code and 43 dB for C/A code in a 50 Hz bandwidth.
Outputs from other navigation sources, (for example inertial navigation system (INS) outputs, radars, altimeters, etc . . . ) are often combined with GPS navigation system outputs to provide navigation solutions which are improved over those resulting from systems which employ either one used independently. These outputs assist in maintaining an accurate navigation solution over limited time intervals during periods of GPS dropout. The INS is also used to aid the GPS receiver in the presence of large platform accelerations, enabling narrow tracking filter bandwidths.
Current GPS-based navigation system architectures can be categorized generally as "loosely coupled" or "tightly coupled". A loosely coupled system, for example, may combine the navigation solution generated by a GPS receiver (position, velocity, time) with the navigation solution provided by an INS navigation system (position, velocity, attitude) using a weighting scheme generally based on a Kalman filter. A minimum of four satellites are required to obtain the GPS navigation solution. A tightly-coupled system computes pseudorange and, deltarange (integral of Doppler velocity) measurements obtained by a GPS receiver, and for example, combines them with the INS navigation solution. A tightly-coupled system offers the advantage of obtaining a navigation solution with less than four satellites in view.
Vulnerability of current GPS receivers to interference has resulted in various designs for interference suppression which generally involve the use of patch antennas and/or specialized signal processors placed in front of the receiver input. Another approach uses aiding signals from inertial and/or other sensors to allow carrier/code tracking at narrow bandwidths in highly dynamic regimes such as aircraft. These methods may be regarded as ad hoc, since they only attempt to modify current systems at easily accessible points within the system. As a result, current systems are far from optimal in terms of interference rejection and navigation accuracy. There is a need for a fully integrated design which achieves near-optimal performance.